Three kinds of rotor tip configurations have been investigated numerically in the LISA 1.5-stage turbine, including the flat tip, the honeycomb tip, and the honeycomb tip with injection. The effect of the cavity depth and the injection mass flowrate on the turbine performance is studied in detail, evaluated by the isentropic total-to-total efficiency and the tip leakage mass flowrate. The Reynolds-averaged Navier–Stokes (RANS) method and the k–ω turbulence model are adopted in all the present computations. The numerical results show that the first-stage efficiency is increased by up to 0.66% and the tip leakage mass flowrate is reduced by about 1.87% of the main flow. The pressure field and the flow feature inside the gap are explored. The flow structures and the total pressure loss contours in the rotor passage are presented. Finally, the total pressure loss is newly defined by considering the injection effect. It is indicated that the injection mass flowrate should be carefully determined for excellent overall performance.